US20050120097A1

US20050120097A1 - Method and apparatus for providing an efficient control channel structure in a wireless communication system

Info

Publication number: US20050120097A1
Application number: US10/725,904
Authority: US
Inventors: J. Walton; John Ketchum
Original assignee: Qualcomm Inc
Current assignee: Qualcomm Inc
Priority date: 2003-12-01
Filing date: 2003-12-01
Publication date: 2005-06-02
Also published as: AU2009213093A1; KR20060095576A; RU2006123443A; IL175904A0; AU2004310960A1; JP4658216B2; BRPI0417069A; RU2332802C2; US9876609B2; US20160270049A1; US9473269B2; JP4409576B2; UA86218C2; KR100834356B1; JP2010050979A; CA2547112A1; AU2009213093B2; WO2005055527A1; AU2004310960C1; AU2004310960B2

Abstract

According to one aspect of the invention, a method is provided in which a control channel used for transmitting control information is partitioned into a plurality of subchannels each of which is operated at a specific data rate. For each of one or more user terminals, one of the subchannels is selected based on one or more selection criteria for transmitting control information from an access point to the respective user terminal. Control information is transmitted from the access point to a user terminal on a particular subchannel selected for the respective user terminal. At the user terminal, one or more subchannels are decoded to obtain control information designated for the user terminal.

Description

RELATED APPLICATIONS

This application is related to U.S. application Ser. No. 10/693,419, entitled “MIMO WLAN System,” filed on Oct. 23, 2003, assigned to the assignee of the present invention.

BACKGROUND

I. Field
The present invention relates generally to data communication and processing, and more specifically to a method and apparatus for providing an efficient control channel structure in a wireless local area network (WLAN) communication system.
II. Background
Wireless communication systems have been widely deployed to provide various types of communication such as voice, packet data, and so on. These systems may be multiple-access systems capable of supporting communication with multiple users sequentially or simultaneously by sharing the available system resources. Examples of multiple-access systems include Code Division Multiple Access (CDMA) systems, Time Division Multiple Access (TDMA) systems, and Frequency Division Multiple Access (FDMA) systems.
In recent years, wireless local area networks (WLANs) have also been widely deployed in accordance with various WLAN standards (e.g., IEEE 802.11a, 802.11b, and 802.11g, etc.) to enable communication among wireless electronic devices (e.g., computers) via wireless link. A WLAN may employ devices called access points (or base stations) that act like hubs and/or routers and provide connectivity for other wireless devices in the network (e.g. user terminals or user stations). The access points may also connect (or “bridge”) the WLAN to wired LANs, thus allowing the wireless devices access to LAN resources.
In a wireless communication system, a radio frequency (RF) modulated signal from a transmitter unit may reach a receiver unit via a number of propagation paths. The characteristics of the propagation paths typically vary over time due to a number of factors, such as fading and multipath. To provide diversity against deleterious path effects and improve performance, multiple transmit and receive antennas may be used. If the propagation paths between the transmit and receive antennas are linearly independent (e.g., a transmission on one path is not formed as a linear combination of the transmissions on the other paths), then the likelihood of correctly receiving a data transmission increases as the number of antennas increases. Generally, diversity increases and performance improves as the number of transmit and receive antennas increases.
A MIMO system employs multiple (N_T) transmit antennas and multiple (N_R) receive antennas for data transmission. A MIMO channel formed by the N_Ttransmit and N_Rreceive antennas may be decomposed into N_Sspatial channels, with N_S≦min{N_T, N_R}. Each of the N_Sspatial channels corresponds to a dimension. The MIMO system can provide improved performance (e.g., increased transmission capacity and/or greater reliability) if the additional dimensionalities created by the multiple transmit and receive antennas are utilized.
An exemplary MIMO WLAN system is described in the aforementioned U.S. patent application Ser. No. 10/693,419, assigned to the assignee of the present invention. Such a MIMO WLAN system may be configured to provide various types of services and support various types of applications, and achieve a high level of system performance. In various embodiments, MIMO and orthogonal frequency division multiplexing (OFDM) may be employed to attain high throughput, combat deleterious path effects, and provide other benefits. Each access point in the system may be configured to support multiple user terminals. The allocation of downlink and uplink resources may be dependent on the requirements of the user terminals, the channel conditions, and other factors.
In one embodiment, the WLAN system as disclosed in the aforementioned U.S. patent application employs a channel structure designed to support efficient downlink and uplink transmissions. Such a channel structure may comprise a number of transport channels that may be used for various functions, such as signaling of system parameters and resource assignments, downlink and uplink data transmissions, random access of the system, and so on. Various attributes of these transport channels may be configurable, which allows the system to easily adapt to changing channel and loading conditions. One of these transport channels, called forward control channel (FCCH), may be used by the access point to allocate resources (e.g., channel assignments) on the downlink and uplink. The FCCH may also be used to provide acknowledgment for messages received on another transport channel.
As disclosed in the aforementioned U.S. patent application, in one embodiment, the FCCH can be transmitted or operable at different data rates (e.g., four different data rates). For example, the different data rates may include 0.25 bps/Hz, 0.5 bps/Hz, 1 bps/Hz, and 2 bps/Hz. However, in such a configuration, the rate employed on the FCCH is dictated by the worst case user in the system (i.e., the user that operates at the lowest data rate). This scheme is inefficient because a single user that cannot operate at a higher rate may reduce the efficiency and utilization of the FCCH, even though other users in the system may be able to operate at higher data rates.
There is, therefore, a need in the art for a method and apparatus to provide a more efficient control channel structure that is able to accommodate different users that may operate at different data rates.

SUMMARY

The various aspects and embodiments of the invention are described in further detail below. According to one aspect of the invention, a method is provided in which a control channel used for transmitting control information is partitioned into a plurality of subchannels each of which is operated at a specific data rate. For each of one or more user terminals, one of the subchannels is selected based on one or more selection criteria for transmitting control information from an access point to the respective user terminal. Control information is transmitted from the access point to a user terminal on a particular subchannel selected for the respective user terminal. At the user terminal, one or more subchannels are decoded to obtain control information designated for the user terminal.

BRIEF DESCRIPTION OF THE DRAWINGS

The various features and aspects of the invention can be understood from the detailed description set forth below in conjunction with the following drawings, in which:
FIG. 1 shows a block diagram of a MIMO WLAN system in which the teachings of the invention are implemented;
FIG. 2 shows a layer structure for the MIMO WLAN system;
FIG. 3 is a block diagram illustrating various components of an access point and user terminals;
FIGS. 4A, 4B and 4C show a TDD-TDM frame structure, an FDD-TDM frame structure, and an FDD-CDM frame structure, respectively;
FIG. 5 shows the TDD-TDM frame structure with five transport channels—BCH, FCCH, FCH, RCH, and RACH;
FIGS. 6A and 6B illustrate various PDU formats for the various transport channels;
FIG. 7 shows a new FCCH structure, in accordance with one embodiment of the invention;
FIG. 8 shows a flow diagram of a method, in accordance with one embodiment of the invention; and
FIG. 9 shows a flow diagram of a decoding process in accordance with one embodiment of the invention.

DETAILED DESCRIPTION

The word “exemplary” is used herein to mean “serving as an example, instance, or illustration.” Any embodiment or design described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments or designs.
FIG. 1 shows a MIMO WLAN system 100 in which the teachings of the present invention are implemented. As shown in FIG. 1, MIMO WLAN system 100 includes a number of access points (APs) 110 that support communication for a number of user terminals (UTs) 120. For simplicity, only two access points 110 are shown in FIG. 1. An access point may also be referred to as a base station, access controller, or communication controller herein.
User terminals 120 may be dispersed throughout the system. Each user terminal may be a fixed or mobile terminal that can communicate with the access point. A user terminal may also be referred to as a mobile station, a remote station, an access terminal, a user equipment (UE), a wireless device, or some other terminology herein. Each user terminal may communicate with one or possibly multiple access points on the downlink and/or uplink at any given moment. The downlink (also called forward link) refers to transmission from the access point to the user terminal, and the uplink (also called reverse link) refers to transmission from the user terminal to the access point.
In FIG. 1, access point 110 a communicates with user terminals 120 a through 120 f, and access point 110 b communicates with user terminals 120 f through 120 k. Depending on the specific design of system 100, an access point may communicate with multiple user terminals simultaneously (e.g., via multiple code channels or subbands) or sequentially (e.g., via multiple time slots). At any given moment, a user terminal may receive downlink transmissions from one or multiple access points. The downlink transmission from each access point may include overhead data intended to be received by multiple user terminals, user-specific data intended to be received by specific user terminals, other types of data, or any combination thereof. The overhead data may include pilot, page and broadcast messages, system parameters, and so on.
In one embodiment, the MIMO WLAN system is based on a centralized controller network architecture. Thus, a system controller 130 couples to access points 110 and may further couple to other systems and networks. For example, system controller 130 may couple to a packet data network (PDN), a wired local area network (LAN), a wide area network (WAN), the Internet, a public switched telephone network (PSTN), a cellular communication network, etc. System controller 130 may be designed to perform a number of functions such as (1) coordination and control for the access points coupled to it, (2) routing of data among these access points, (3) access and control of communication with the user terminals served by these access points, and so on. The MIMO WLAN system as shown in FIG. 1 may be operated in various frequency bands (e.g., the 2.4 GHz and 5.33 GHz U-NII bands), subject to the bandwidth and emission constraints specific to the selected operating band.
In one embodiment, each access point may be equipped with multiple transmit and receive antennas (e.g., four transmit and receive antennas) for data transmission and reception. Each user terminal may be equipped with a single transmit/receive antenna or multiple transmit/receive antennas for data transmission and reception. The number of antennas employed by each user terminal type may be dependent on various factors such as, for example, the services to be supported by the user terminal (e.g., voice, data, or both), cost considerations, regulatory constraints, safety issues, and so on.
For a given pairing of multi-antenna access point and multi-antenna user terminal, a MIMO channel is formed by the N_Ttransmit antennas and N_Rreceive antennas available for use for data transmission. Different MIMO channels are formed between the access point and different multi-antenna user terminals. Each MIMO channel may be decomposed into N_Sspatial channels, with N_S≦min{N_T, N_R}. N_Sdata streams may be transmitted on the Ns spatial channels. Spatial processing is required at a receiver and may or may not be performed at a transmitter in order to transmit multiple data streams on the N_Sspatial channels.
The N_Sspatial channels may or may not be orthogonal to one another. This depends on various factors such as (1) whether or not spatial processing was performed at the transmitter to obtain orthogonal spatial channels and (2) whether or not the spatial processing at both the transmitter and the receiver was successful in orthogonalizing the spatial channels. If no spatial processing is performed at the transmitter, then the N_Sspatial channels may be formed with N_Stransmit antennas and are unlikely to be orthogonal to one another.
The N_Sspatial channels may be orthogonalized by performing decomposition on a channel response matrix for the MIMO channel, as described in the aforementioned U.S. patent application. For a given number of (e.g., four) antennas at the access point, the number of spatial channels available for each user terminal is dependent on the number of antennas employed by that user terminal and the characteristics of the wireless MIMO channel that couples the access point antennas and the user terminal antennas. If a user terminal is equipped with one antenna, then the four antennas at the access point and the single antenna at the user terminal form a multiple-input single-output (MISO) channel for the downlink and a single-input multiple-output (SIMO) channel for the uplink.

The MIMO WLAN system as shown in FIG. 1 may be designed and configured to support various transmission modes, as illustrated in Table 1 below.

TABLE 1


Transmission
modes	Description

SIMO	Data is transmitted from a single antenna but may be
	received by multiple antennas for receive diversity.
Diversity	Data is redundantly transmitted from multiple transmit
	antennas and/or multiple subbands to provide diversity.
Beam-steering	Data is transmitted on a single (best) spatial channel
	at full power using phase steering information for the
	principal eigenmode of the MIMO channel.
Spatial multi-	Data is transmitted on multiple spatial channels to
plexing	achieve higher spectral efficiency.

The transmission modes available for use for the downlink and uplink for each user terminal are dependent on the number of antennas employed at the user terminal. Table 2 lists the transmission modes available for different terminal types for the downlink and uplink, assuming multiple (e.g., four) antennas at the access point.

	TABLE 2


	Downlink	Uplink

	Single-	Multi-	Single-	Multi-
Transmission	antenna user	antenna user	antenna user	antenna user
modes	terminal	terminal	terminal	terminal

MISO (on	X	X	X	X
downlink)/
SIMO (on
uplink)
Diversity	X	X		X
Beam-steering	X	X		X
Spatial multi-		X		X
plexing

In an embodiment, the MIMO WLAN system employs OFDM to effectively partition the overall system bandwidth into a number of (N_F) orthogonal subbands. These subbands are also referred to as tones, bins, or frequency channels. With OFDM, each subband is associated with a respective subcarrier that may be modulated with data. For a MIMO system that utilizes OFDM, each spatial channel of each subband may be viewed as an independent transmission channel where the complex gain associated with each subband is effectively constant across the subband bandwidth.
In one embodiment, the system bandwidth can be partitioned into 64 orthogonal subbands (i.e., N_F=64), which are assigned indices of −32 to +31. Of these 64 subbands, 48 subbands (e.g., with indices of ±{1, . . . , 6, 8, . . . , 20, 22, . . . , 26}) can be used for data, 4 subbands (e.g., with indices of ±{7, 21}) can be used for pilot and possibly signaling, the DC subband (with index of 0) is not used, and the remaining subbands are also not used and serve as guard subbands. This OFDM subband structure is described in further detail in a document for IEEE Standard 802.11a and entitled “Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications: High-speed Physical Layer in the 5 GHz Band,” September 1999, which is publicly available. In other embodiments, different numbers of subbands and various other OFDM subband structures may also be implemented for the MIMO WLAN system. For example, all 53 subbands with indices from −26 to +26 may be used for data transmission. As another example, a 128-subband structure, a 256-subband structure, or a subband structure with some other number of subbands may be used.
For OFDM, the data to be transmitted on each subband is first modulated (i.e., symbol mapped) using a particular modulation scheme selected for use for that subband. Zeros are provided for the unused subbands. For each symbol period, the modulation symbols and zeros for all N_Fsubbands are transformed to the time domain using an inverse fast Fourier transform (IFFT) to obtain a transformed symbol that contains N_Ftime-domain samples. The duration of each transformed symbol is inversely related to the bandwidth of each subband. In one specific design for the MIMO WLAN system, the system bandwidth is 20 MHz, N_F=64, the bandwidth of each subband is 312.5 KHz, and the duration of each transformed symbol is 3.2 μsec.
OFDM can provide certain advantages, such as the ability to combat frequency selective fading, which is characterized by different channel gains at different frequencies of the overall system bandwidth. It is well known that frequency selective fading causes inter-symbol interference (ISI), which is a phenomenon whereby each symbol in a received signal acts as distortion to subsequent symbols in the received signal. The ISI distortion degrades performance by impacting the ability to correctly detect the received symbols. Frequency selective fading can be conveniently combated with OFDM by repeating a portion of (or appending a cyclic prefix to) each transformed symbol to form a corresponding OFDM symbol, which is then transmitted.
The length of the cyclic prefix (i.e., the amount to repeat) for each OFDM symbol is dependent on the delay spread of the wireless channel. In particular, to effectively combat ISI, the cyclic prefix should be longer than the maximum expected delay spread for the system.
In an embodiment, cyclic prefixes of different lengths may be used for the OFDM symbols, depending on the expected delay spread. For the MIMO WLAN system described above, a cyclic prefix of 400 nsec (8 samples) or 800 nsec (16 samples) may be selected for use for the OFDM symbols. A “short” OFDM symbol uses the 400 nsec cyclic prefix and has a duration of 3.6 μsec. A “long” OFDM symbol uses the 800 nsec cyclic prefix and has a duration of 4.0 μsec. Short OFDM symbols may be used if the maximum expected delay spread is 400 nsec or less, and long OFDM symbols may be used if the delay spread is greater than 400 nsec. Different cyclic prefixes may be selected for use for different transport channels, and the cyclic prefix may also be dynamically selectable, as described below. Higher system throughput may be achieved by using the shorter cyclic prefix when possible, since more OFDM symbols of shorter duration can be transmitted over a given fixed time interval.
FIG. 2 illustrates a layer structure 200 that may be used for the MIMO WLAN system. As shown in FIG. 2, in one embodiment, layer structure 200 includes (1) applications and upper layer protocols that approximately correspond to Layer 3 and higher of the ISO/OSI reference model (upper layers), (2) protocols and services that correspond to Layer 2 (the link layer), and (3) protocols and services that correspond to Layer 1 (the physical layer).
The upper layers includes various applications and protocols, such as signaling services 212, data services 214, voice services 216, circuit data applications, and so on. Signaling is typically provided as messages and data is typically provided as packets. The services and applications in the upper layers originate and terminate messages and packets according to the semantics and timing of the communication protocol between the access point and the user terminal. The upper layers utilize the services provided by Layer 2.
Layer 2 supports the delivery of messages and packets generated by the upper layers. In the embodiment shown in FIG. 2, Layer 2 includes a Link Access Control (LAC) sublayer 220 and a Medium Access Control (MAC) sublayer 230. The LAC sublayer implements a data link protocol that provides for the correct transport and delivery of messages generated by the upper layers. The LAC sublayer utilizes the services provided by the MAC sublayer and Layer 1. The MAC sublayer is responsible for transporting messages and packets using the services provided by Layer 1. The MAC sublayer controls the access to Layer 1 resources by the applications and services in the upper layers. The MAC sublayer may include a Radio Link Protocol (RLP) 232, which is a retransmission mechanism that may be used to provide higher reliability for packet data. Layer 2 provides protocol data units (PDUs) to Layer 1.
Layer 1 comprises physical layer 240 and supports the transmission and reception of radio signals between the access point and user terminal. The physical layer performs coding, interleaving, modulation, and spatial processing for various transport channels used to send messages and packets generated by the upper layers. In this embodiment, the physical layer includes a multiplexing sublayer 242 that multiplexes processed PDUs for various transport channels into the proper frame format. Layer 1 provides data in units of frames.
It should be understood by one skilled in the art that various other suitable layer structures may also be designed and used for the MIMO WLAN system.
FIG. 3 shows a block diagram of one embodiment of an access point 110 x and two user terminals 120 x and 120 y within the MIMO WLAN system.
On the downlink, at access point 110 x, a transmit (TX) data processor 310 receives traffic data (e.g., information bits) from a data source 308 and signaling and other information from a controller 330 and possibly a scheduler 334. These various types of data may be sent on different transport channels that are described in more details below. TX data processor 310 “frames” the data (if necessary), scrambles the framed/unframed data, encodes the scrambled data, interleaves (i.e., reorders) the coded data, and maps the interleaved data into modulation symbols. For simplicity, a “data symbol” refers to a modulation symbol for traffic data, and a “pilot symbol” refers to a modulation symbol for pilot. The scrambling randomizes the data bits. The encoding increases the reliability of the data transmission. The interleaving provides time, frequency, and/or spatial diversity for the code bits. The scrambling, coding, and modulation may be performed based on control signals provided by controller 330. TX data processor 310 provides a stream of modulation symbols for each spatial channel used for data transmission.
A TX spatial processor 320 receives one or more modulation symbol streams from TX data processor 310 and performs spatial processing on the modulation symbols to provide four streams of transmit symbols, one stream for each transmit antenna.
Each modulator (MOD) 322 receives and processes a respective transmit symbol stream to provide a corresponding stream of OFDM symbols. Each OFDM symbol stream is further processed to provide a corresponding downlink modulated signal. The four downlink modulated signals from modulator 322 a through 322 d are then transmitted from four antennas 324 a through 324 d, respectively.
At each user terminal 120, one or multiple antennas 352 receive the transmitted downlink modulated signals, and each receive antenna provides a received signal to a respective demodulator (DEMOD) 354. Each demodulator 354 performs processing complementary to that performed at modulator 322 and provides received symbols. A receive (RX) spatial processor 360 then performs spatial processing on the received symbols from all demodulators 354 to provide recovered symbols, which are estimates of the modulation symbols sent by the access point.
An RX data processor 370 receives and demultiplexes the recovered symbols into their respective transport channels. The recovered symbols for each transport channel may be symbol demapped, deinterleaved, decoded, and descrambled to provide decoded data for that transport channel. The decoded data for each transport channel may include recovered packet data, messages, signaling, and so on, which are provided to a data sink 372 for storage and/or a controller 380 for further processing.
For the downlink, at each active user terminal 120, RX spatial processor 360 further estimates the downlink to obtain channel state information (CSI). The CSI may include channel response estimates, received SNRs, and so on. RX data processor 370 may also provide the status of each packet/frame received on the downlink. A controller 380 receives the channel state information and the packet/frame status and determines the feedback information to be sent back to the access point. The feedback information is processed by a TX data processor 390 and a TX spatial processor 392 (if present), conditioned by one or more modulators 354, and transmitted via one or more antennas 352 back to the access point.
At access point 110, the transmitted uplink signal(s) are received by antennas 324, demodulated by demodulators 322, and processed by an RX spatial processor 340 and an RX data processor 342 in a complementary manner to that performed at the user terminal. The recovered feedback information is then provided to controller 330 and a scheduler 334.
In one embodiment, scheduler 334 uses the feedback information to perform a number of functions such as (1) selecting a set of user terminals for data transmission on the downlink and uplink, (2) selecting the transmission rate(s) and the transmission mode for each selected user terminal, and (3) assigning the available FCH/RCH resources to the selected terminals. Scheduler 334 and/or controller 330 further uses information (e.g., steering vectors) obtained from the uplink transmission for the processing of the downlink transmission.

As mentioned above, a number of services and applications may be supported by the MIMO WLAN system and various transport channels may be defined for the MIMO WLAN system to carry various types of data. Table 3 lists an exemplary set of transport channels and also provides a brief description for each transport channel.

TABLE 3


Transport channels	Description

Broadcast	BCH	Used by the access point to transmit pilot
channel		and system parameters to the user terminals.
Forward	FCCH	Used by the access point to allocate re-
control		sources on the downlink and uplink. The
channel		resource allocation may be performed on a
		frame-by-frame basis. Also used to provide
		acknowledgment for messages received on the
		RACH.
Forward	FCH	Used by the access point to transmit user-
channel		specific data to the user terminals and
		possibly a reference (pilot) used by the
		user terminals for channel estimation. May
		also be used in a broadcast mode to send
		page and broadcast messages to multiple user
		terminals.
Random	RACH	Used by the user terminals to gain access to
access		the system and send short messages to the
channel		access point.
Reverse	RCH	Used by the user terminals to transmit data
channel		to the access point. May also carry a
		reference used by the access point for
		channel estimation.

As shown in Table 3, the downlink transport channels used by the access point includes the BCH, FCCH, and FCH. The uplink transport channels used by the user terminals include the RACH and RCH. It should be recognized by one skilled in the art that the transport channels listed in Table 3 represent an exemplary embodiment of a channel structure that may be used for the MIMO WLAN system. Fewer, additional, and/or different transport channels may also be defined for use for the MIMO WLAN system. For example, certain functions may be supported by function-specific transport channels (e.g., pilot, paging, power control, and sync channel channels). Thus, other channel structures with different sets of transport channels may be defined and used for the MIMO WLAN system, within the scope of the invention.
A number of frame structures may be defined for the transport channels. The specific frame structure to use for the MIMO WLAN system is dependent on various factors such as, for example, (1) whether the same or different frequency bands are used for the downlink and uplink and (2) the multiplexing scheme used to multiplex the transport channels together.
If only one frequency band is available, then the downlink and uplink may be transmitted on different phases of a frame using time division duplexing (TDD). If two frequency bands are available, then the downlink and uplink may be transmitted on different frequency bands using frequency division duplexing (FDD).
For both TDD and FDD, the transport channels may be multiplexed together using time division multiplexing (TDM), code division multiplexing (CDM), frequency division multiplexing (FDM), and so on. For TDM, each transport channel is assigned to a different portion of a frame. For CDM, the transport channels are transmitted concurrently but each transport channel is channelized by a different channelization code, similar to that performed in a code division multiple access (CDMA) system. For FDM, each transport channel is assigned a different portion of the frequency band for the link.
Table 4 lists the various frame structures that may be used to carry the transport channels. Each of these frame structures is described in further detail below.

TABLE 4

Shared frequency band for Separate frequency bands

downlink and uplink for downlink and uplink

Time division TDD-TDM frame structure FDD-TDM frame structure

Code division TDD-CDM frame structure FDD-CDM frame structure
FIG. 4A illustrates an embodiment of a TDD-TDM frame structure 400 a that may be used if a single frequency band is used for both the downlink and uplink. Data transmission occurs in units of TDD frames. Each TDD frame may be defined to span a particular time duration. The frame duration may be selected based on various factors such as, for example, (1) the bandwidth of the operating band, (2) the expected sizes of the PDUs for the transport channels, and so on. In general, a shorter frame duration may provide reduced delays. However, a longer frame duration may be more efficient since header and overhead may represent a smaller fraction of the frame. In one embodiment, each TDD frame has a duration of 2 msec.
As shown in FIG. 4A, each TDD frame can be partitioned into a downlink phase and an uplink phase. The downlink phase is further partitioned into three segments for the three downlink transport channels—the BCH, FCCH, and FCH. The uplink phase is further partitioned into two segments for the two uplink transport channels—the RCH and RACH.
The segment for each transport channel may be defined to have either a fixed duration or a variable duration that can change from frame to frame. In one embodiment, the BCH segment is defined to have a fixed duration, and the FCCH, FCH, RCH, and RACH segments are defined to have variable durations.
The segment for each transport channel may be used to carry one or more protocol data units (PDUs) for that transport channel. In the embodiment shown in FIG. 4A, a BCH PDU is transmitted in a first segment 410, an FCCH PDU is transmitted in a second segment 420, and one or more FCH PDUs are transmitted in a third segment 430 of the downlink phase. On the uplink phase, one or more RCH PDUs are transmitted in a fourth segment 440 and one or more RACH PDUs are transmitted in a fifth segment 450 of the TDD frame.
Frame structure 400 a represents one arrangement of the various transport channels within a TDD frame. This arrangement can provide certain benefits such as reduced delays for data transmission on the downlink and uplink. The BCH is transmitted first in the TDD frame since it carries system parameters that may be used for the PDUs of the other transport channels within the same TDD frame. The FCCH is transmitted next since it carries resource allocation (e.g., channel assignment) information indicative of which user terminal(s) are designated to receive downlink data on the FCH and which user terminal(s) are designated to transmit uplink data on the RCH within the current TDD frame. Other TDD-TDM frame structures may also be defined and used for the MIMO WLAN system.
FIG. 4B illustrates an embodiment of an FDD-TDM frame structure 400 b that may be used if the downlink and uplink are transmitted using two separate frequency bands. Downlink data is transmitted in a downlink frame 402 a, and uplink data is transmitted in an uplink frame 402 b. Each downlink and uplink frame may be defined to span a particular time duration (e.g., 2 msec). For simplicity, the downlink and uplink frames may be defined to have the same duration and may further be defined to be aligned at the frame boundaries. However, different frame durations and/or non-aligned (i.e., offset) frame boundaries may also be used for the downlink and uplink.
As shown in FIG. 4B, the downlink frame is partitioned into three segments for the three downlink transport channels. The uplink frame is partitioned into two segments for the two uplink transport channels. The segment for each transport channel may be defined to have a fixed or variable duration, and may be used to carry one or more PDUs for that transport channel.
In the embodiment shown in FIG. 4B, the downlink frame carries a BCH PDU, an FCCH PDU, and one or more FCH PDUs in segments 410, 420, and 430, respectively. The uplink frame carries one or more RCH PDUs and one or more RACH PDUs in segments 440 and 450, respectively. This arrangement may provide the benefits described above (e.g., reduced delays for data transmission). Other FDD-TDM frame structures may also be defined and used for the MIMO WLAN system, and this is within the scope of the invention.
FIG. 4C illustrates an embodiment of an FDD-CDM/FDM frame structure 400 c that may also be used if the downlink and uplink are transmitted using separate frequency bands. Downlink data may be transmitted in a downlink frame 404 a, and uplink data may be transmitted in an uplink frame 404 b. The downlink and uplink frames may be defined to have the same duration (e.g., 2 msec) and aligned at the frame boundaries.
As shown in FIG. 4C, the three downlink transport channels are transmitted concurrently in the downlink frame, and the two uplink transport channels are transmitted concurrently in the uplink frame. For CDM, the transport channels for each link are “channelized” with different channelization codes, which may be Walsh codes, orthogonal variable spreading factor (OVSF) codes, quasi-orthogonal functions (QOF), and so on. For FDM, the transport channels for each link are assigned different portions of the frequency band for the link. Different amounts of transmit power may also be used for different transport channels in each link.
Other frame structures may also be defined for the downlink and uplink transport channels, and this is within the scope of the invention. Moreover, it is possible to use different types of frame structure for the downlink and uplink. For example, a TDM-based frame structure may be used for the downlink and a CDM-based frame structure may be used for the uplink.
In one embodiment, the transport channels as described above are used to send various types of data and may be categorized into two groups: common transport channels and dedicated transport channels.
The common transport channels, in one embodiment, may include the BCH, FCCH, and RACH. These transport channels are used to send data to or receive data from multiple user terminals. The BCH and FCCH can be transmitted by the access point using the diversity mode. On the uplink, the RACH can be transmitted by the user terminals using the beam-steering mode (if supported by the user terminal). The BCH can be operated at a known fixed rate so that the user terminals can receive and process the BCH without any additional information. As described in more details below, the FCCH support multiple rates to allow for greater efficiency. Each “rate” or “rate set” may be associated with a particular code rate (or coding scheme) and a particular modulation scheme.
The dedicated transport channels, in one embodiment, include the FCH and RCH. These transport channels are normally used to send user-specific data to or by specific user terminals. The FCH and RCH may be dynamically allocated to the user terminals as necessary and as available. The FCH may also be used in a broadcast mode to send overhead, page, and broadcast messages to the user terminals. In general, the overhead, page, and broadcast messages are transmitted prior to any user-specific data on the FCH.
FIG. 5 illustrates an exemplary transmission on the BCH, FCCH, FCH, RCH, and RACH based on TDD-TDM frame structure 400 a. In this embodiment, one BCH PDU 510 and one FCCH PDU 520 are transmitted in BCH segment 410 and FCCH segment 420, respectively. FCH segment 430 may be used to send one or more FCH PDUs 530, each of which may be intended for a specific user terminal or multiple user terminals. Similarly, one or more RCH PDUs 540 may be sent by one or more user terminals in RCH segment 440. The start of each FCH/RCH PDU is indicated by an FCH/RCH offset from the end of the preceding segment. A number of RACH PDUs 550 may be sent in RACH segment 450 by a number of user terminals to access the system and/or to send short messages.
In one embodiment, the BCH is used by the access point to transmit a beacon pilot, a MIMO pilot, and system parameters to the user terminals. The beacon pilot is used by the user terminals to acquire system timing and frequency. The MIMO pilot is used by the user terminals to estimate the MIMO channel formed by the access point antennas and their own antennas. The system parameters specify various attributes of the downlink and uplink transmissions. For example, since the durations of the FCCH, FCH, RACH, and RCH segments are variable, the system parameters that specify the length of each of these segments for the current TDD frame are sent in the BCH.
FIG. 6A illustrates an embodiment of BCH PDU 410. In this embodiment, BCH PDU 410 includes a preamble portion 510 and a message portion 516. Preamble portion 510 further includes a beacon pilot portion 512 and a MIMO pilot portion 514. Portion 512 carries a beacon pilot and has a fixed duration of T_CP=8 μsec. Portion 514 carries a MIMO pilot and has a fixed duration of T_MP=32 μsec. Portion 516 carries a BCH message and has a fixed duration of T_BM=40 μsec. A preamble may be used to send one or more types of pilot and/or other information. A beacon pilot comprises a specific set of modulation symbols that is transmitted from all transmit antennas. A MIMO pilot comprises a specific set of modulation symbols that is transmitted from all transmit antennas with different orthogonal codes, which then allows the receivers to recover the pilot transmitted from each antenna. Different sets of modulation symbols may be used for the beacon and MIMO pilots.

In one embodiment, the BCH message carries system configuration information. Table 5 lists the various fields for an exemplary BCH message format.

TABLE 5


BCH Message

Fields/
Parameter	Length
Names	(bits)	Description

Frame	4	TDD frame counter
Counter
Net ID	10	Network identifier (ID)
AP ID	6	Access point ID
AP Tx Lvl	4	Access point transmit level
AP Rx Lvl	3	Access point receive level
FCCH Length	6	Duration of FCCH (in units of OFDM
		symbols)
FCCH Rate	2	Physical layer rate of FCCH
FCH Length	9	Duration of FCH (in units of OFDM symbols)
RCH Length	9	Duration of RCH (in units of OFDM symbols)
RACH Length	5	Duration of RACH (in units of RACH slots)
RACH Slot	2	Duration of each RACH slot (in units of
Size		OFDM symbols)
RACH Guard	2	Guard interval at the end of RACH
Interval
Cyclic Pre-	1	Cyclic prefix duration
fix Duration
Page Bit
	1	“0” = page message sent on FCH
		“1” = no page message sent
Broadcast	1	“0” = broadcast message sent on FCH
Bit		“1” = no broadcast message sent
RACH	1	“0” = RACH acknowledgment sent on FCH
Acknowledg-		“1” = no RACH acknowledgment sent
ment Bit
CRC	16	CRC value for the BCH message
Tail Bits	6	Tail bits for convolutional encoder
Reserved	32	Reserved for future use

The Frame Counter value may be used to synchronize various processes at the access point and user terminals (e.g., the pilot, scrambling codes, cover code, and so on). A frame counter may be implemented with a 4-bit counter that wraps around. This counter is incremented at the start of each TDD frame, and the counter value is included in the Frame Counter field. The Net ID field indicates the identifier (ID) of the network to which the access point belongs. The AP ID field indicates the ID of the access point within the network ID. The AP Tx Lvl and AP Rx Lvl fields indicate the maximum transmit power level and the desired receive power level at the access point, respectively. The desired receive power level may be used by the user terminal to determine the initial uplink transmit power.
The FCCH Length, FCH Length, and RCH Length fields indicate the lengths of the FCCH, FCH, and RCH segments, respectively, for the current TDD frame. In one embodiment, the lengths of these segments are given in units of OFDM symbols. The OFDM symbol duration for the BCH can be fixed at 4.0 μsec. The OFDM symbol duration for all other transport channels (e.g., the FCCH, FCH, RACH, and RCH) is variable and depends on the selected cyclic prefix, which is specified by the Cyclic Prefix Duration field. The FCCH Rate field indicates the rate used for the FCCH for the current TDD frame.
The RACH Length field indicates the length of the RACH segment, which is given in units of RACH slots. The duration of each RACH slot is given by the RACH Slot Size field, in units of OFDM symbols. The RACH Guard Interval field indicates the amount of time between the last RACH slot and the start of the BCH segment for the next TDD frame.
The Page Bit and Broadcast Bit indicate whether or not page messages and broadcast messages, respectively, are being sent on the FCH in the current TDD frame.
These two bits may be set independently for each TDD frame. The RACH Acknowledgment Bit indicates whether or not acknowledgments for PDUs sent on the RACH in prior TDD frames are being sent on the FCCH in the current TDD frame.
The CRC field includes a CRC value for the entire BCH message. This CRC value may be used by the user terminals to determine whether the received BCH message is decoded correctly or in error. The Tail Bits field includes a group of zeros used to reset the convolutional encoder to a known state at the end of the BCH message.
As shown in Table 5, the BCH message includes a total of 120 bits. These 120 bits may be transmitted with 10 OFDM symbols. Table 5 shows one embodiment of the format for the BCH message. Other BCH message formats with fewer, additional, and/or different fields may also be defined and used, and this is within the scope of the invention.
In one embodiment, the access point may allocate resources for the FCH and RCH on a per frame basis. The FCCH is used by the access point to convey the resource allocation information for the FCH and RCH (e.g., the channel assignments).
FIG. 6B illustrates an embodiment of FCCH PDU 420. In this embodiment, the FCCH PDU includes only a portion 520 for an FCCH message. The FCCH message has a variable duration that can change from frame to frame, depending on the amount of scheduling information being carried on the FCCH for that frame. The FCCH message duration is in even number of OFDM symbols and given by the FCCH Length field on the BCH message. The duration of messages sent using the diversity mode (e.g., BCH and FCCH messages) is given in even number of OFDM symbols because the diversity mode transmits OFDM symbols in pairs.
In an embodiment, the FCCH can be transmitted using four possible rates. The specific rate used for the FCCH PDU in each TDD frame is indicated by the FCCH Phy Mode field in the BCH message. Each FCCH rate corresponds to a particular code rate and a particular modulation scheme and is further associated with a particular transmission mode.

An FCCH message may include zero, one, or multiple information elements (IEs). Each information element may be associated with a specific user terminal and may be used to provide information indicative of the assignment of FCH/RCH resources for that user terminal. Table 6 lists the various fields for an exemplary FCCH message format.

TABLE 6


FCCH Message

Fields/
Parameter	Length
Names	(bits)	Description

N_IE

6

Number of IEs included in the FCCH message

N_IE information elements, each including:

IE Type	4	IE type
MAC ID	10	ID assigned to the user terminal
Control	48 or 72	Control fields for channel assignment
Fields
Padding	Variable	Pad bits to achieve even number of OFDM
Bits		symbols in the FCCH message
CRC	16	CRC value for the FCCH message
Tail Bits	6	Tail bits for convolutional encoder

The N_IE field indicates the number of information elements included in the FCCH message sent in the current TDD frame. For each information element (IE) included in the FCCH message, the IE Type field indicates the particular type of this IE.
Various IE types are defined for use to allocate resources for different types of transmissions, as described below.
The MAC ID field identifies the specific user terminal for which the information element is intended. Each user terminal registers with the access point at the start of a communication session and is assigned a unique MAC ID by the access point. This MAC ID is used to identify the user terminal during the session.
The Control Fields are used to convey channel assignment information for the user terminal and are described in detail below. The Padding Bits field includes a sufficient number of padding bits so that the overall length of the FCCH message is an even number of OFDM symbols. The FCCH CRC field includes a CRC value that may be used by the user terminals to determine whether the received FCCH message is decoded correctly or in error. The Tail Bits field includes zeros used to reset the conventional encoder to a known state at the end of the FCCH message. Some of these fields are described in further detail below.

A number of transmission modes are supported by the MIMO WLAN system for the FCH and RCH, as indicated in Table 1. Moreover, a user terminal may be active or idle during a connection. Thus, a number of types of IE are defined for use to allocate FCH/RCH resources for different types of transmissions. Table 7 lists an exemplary set of IE types.

TABLE 7


FCCH IE Types

	IE Size
IE Type	(bits)	IE Type	Description

0	48	Diversity Mode	Diversity mode only
1	72	Spatial Multiplexing	Spatial multiplexing mode -
		Mode	variable rate services
2	48	Idle Mode	Idle state - variable rate
			services

3	48	RACH Acknowledgment	RACH acknowledgment -
			diversity mode
4		Beam Steering Mode	Beam steering mode
5-15	—	Reserved	Reserved for future use

For IE types 0, 1 and 4, resources are allocated to a specific user terminal for both the FCH and RCH (i.e., in channel pairs). For IE type 2, minimal resources are allocated to the user terminal on the FCH and RCH to maintain up-to-date estimate of the link. An exemplary format for each IE type is described below. In general, the rates and durations for the FCH and RCH can be independently assigned to the user terminals.

IE type 0 and 4 are used to allocate FCH/RCH resources for the diversity and beam-steering modes, respectively. For fixed low-rate services (e.g., voice), the rate remains fixed for the duration of the call. For variable rate services, the rate may be selected independently for the FCH and RCH. The FCCH IE indicates the location of the FCH and RCH PDUs assigned to the user terminal. Table 8 lists the various fields of an exemplary IE Type 0 and 4 information element.

TABLE 8


FCCH IE Type 0 and 4

Fields/
Parameter	Length
Names	(bits)	Description

IE Type	4	IE type
MAC ID	10	Temporary ID assigned to the user terminal
FCH Offset	9	FCH offset from start of the TDD frame
		(in OFDM symbols)
FCH Preamble	2	FCH preamble size (in OFDM symbols)
Type
FCH Rate	4	Rate for the FCH
RCH Offset	9	RCH offset from start of the TDD frame
		(in OFDM symbols)
RCH Preamble	2	RCH preamble size (in OFDM symbols)
Type
RCH Rate	4	Rate for the RCH
RCH Timing
	2	Timing adjustment parameter for RCH
Adjustment
RCH Power
	2	Power control bits for RCH
Control

The FCH and RCH Offset fields indicate the time offset from the beginning of the current TDD frame to the start of the FCH and RCH PDUs, respectively, assigned by the information element. The FCH and RCH Rate fields indicate the rates for the FCH and RCH, respectively.
The FCH and RCH Preamble Type fields indicate the size of the preamble in the FCH and RCH PDUs, respectively. Table 9 lists the values for the FCH and RCH Preamble Type fields and the associated preamble sizes.

TABLE 9

Preamble Type

Type Bits Preamble Size

0 00 0 OFDM symbol

1 01 1 OFDM symbol

2 10 4 OFDM symbols

3 11 8 OFDM symbols
The RCH Timing Adjustment field includes two bits used to adjust the timing of the uplink transmission from the user terminal identified by the MAC ID field. This timing adjustment is used to reduce interference in a TDD-based frame structure where the downlink transmissions are time division duplexed. Table 10 lists the values for the RCH Timing Adjustment field and the associated actions.

TABLE 10

RCH Timing Adjustment

Bits Description

00 Maintain current timing

01 Advance uplink transmit timing by 1 sample

10 Delay uplink transmit timing by 1 sample

11 Not used

The RCH Power Control field includes two bits used to adjust the transmit power of the uplink transmission from the identified user terminal. This power control is used to reduce interference on the uplink. Table 11 lists the values for the RCH Power Control field the associated actions.

TABLE 11


RCH Power Control

Bits	Description

00	Maintain current transmit power
01	Increase uplink transmit power by δdB, where
	δ is a system parameter.
10	Decrease uplink transmit power by δdB,
	where δ is a system parameter.
11	Not used

The channel assignment for the identified user terminal may be provided in various manners. In an embodiment, the user terminal is assigned FCH/RCH resources for only the current TDD frame. In another embodiment, the FCH/RCH resources are asigned to the terminal for each TDD frame until canceled. In yet another embodiment, the FCH/RCH resources are assigned to the user terminal for every n-th TDD frame, which is referred to as “decimated” scheduling of TDD frames. The different types of assignment may be indicated by an Assignment Type field in the FCCH information element.

IE type 1 is used to allocate FCH/RCH resources to user terminals using the spatial multiplexing mode. The rate for these user terminals is variable, and may be selected independently for the FCH and RCH. Table 12 lists the various fields of an exemplary IE type 1 information element.

TABLE 12


FCCH IE Type 1

Fields/
Parameter	Length
Names	(bits)	Description

IE Type	4	IE type
MAC ID	10	Temporary ID assigned to the user terminal
FCH Offset	9	FCH offset from end of FCCH
		(in OFDM symbols)
FCH Preamble	2	FCH preamble size (in OFDM symbols)
Type
FCH Spatial	4	Rate for the FCH for spatial channel 1
Channel 1 Rate
FCH Spatial	4	Rate for the FCH for spatial channel 2
Channel 2 Rate
FCH Spatial	4	Rate for the FCH for spatial channel 3
Channel 3 Rate
FCH Spatial	4	Rate for the FCH for spatial channel 4
Channel 4 Rate
RCH Offset	9	RCH offset from end of FCH
		(in OFDM symbols)
RCH Preamble	2	RCH preamble size (in OFDM symbols)
Type
RCH Spatial	4	Rate for the RCH for spatial channel 1
Channel 1 Rate
RCH Spatial	4	Rate for the RCH for spatial channel 2
Channel 2 Rate
RCH Spatial	4	Rate for the RCH for spatial channel 3
Channel 3 Tate
RCH Spatial	4	Rate for the RCH for spatial channel 4
Channel 4 Rate
RCH Timing
	2	Timing adjustment parameter for RCH
Adjustment
Reserved
	2	Reserved for future use

For IE type 1, the rate for each spatial channel may be selected independently on the FCH and RCH. The interpretation of the rates for the spatial multiplexing mode is general in that it can specify the rate per spatial channel (e.g., for up to four spatial channels for the embodiment shown in Table 12). The rate is given per eigenmode if the transmitter performs spatial processing to transmit data on the eigenmodes. The rate is given per antenna if the transmitter simply transmits data from the transmit antennas and the receiver performs the spatial processing to isolate and recover the data (for the non-steered spatial multiplexing mode).
The information element includes the rates for all enabled spatial channels and zeros for the ones not enabled. User terminals with less than four transmit antennas set the unused FCH/RCH Spatial Channel Rate fields to zero. Since the access point is equipped with four transmit/receive antennas, user terminals with more than four transmit antennas may use them to transmit up to four independent data streams.

IE type 2 is used to provide control information for user terminals operating in an Idle state. In an embodiment, when a user terminal is in the Idle state, steering vectors used by the access point and user terminal for spatial processing are continually updated so that data transmission can start quickly if and when resumed. Table 13 lists the various fields of an exemplary IE type 2 information element.

TABLE 13


FCCH IE Type 2

Fields/
Parameter	Length
Names	(bits)	Description

IE Type	4	IE type
MAC ID	10	Temporary ID assigned to the user
		terminal
FCH Offset	9	FCH offset from end of FCCH (in OFDM
		symbols)
FCH Preamble	2	FCH preamble size (in OFDM symbols)
Type
RCH Offset	9	RCH offset from end of FCH (in OFDM
		symbols)
RCH Preamble	2	RCH preamble size (in OFDM symbols)
Type
Reserved	12	Reserved for future use

IE type 3 is used to provide quick acknowledgment for user terminals attempting to access the system via the RACH. To gain access to the system or to send a short message to the access point, a user terminal may transmit an RACH PDU on the uplink.
After the user terminal sends the RACH PDU, it monitors the BCH to determine if the RACH Acknowledgement Bit is set. This bit is set by the access point if any user terminal was successful in accessing the system and an acknowledgment is being sent for at least one user terminal on the FCCH. If this bit is set, then the user terminal processes the FCCH for acknowledgment sent on the FCCH. IE Type 3 information elements are sent if the access point desires to acknowledge that it correctly decoded the RACH PDUs from the user terminals without assigning resources. Table 14 lists the various fields of an exemplary IE Type 3 information element.

TABLE 14

FCCH IE Type 3

Fields/ Length

Parameter Names (bits) Description

IE Type 4 IE type

MAC ID 10 Temporary ID assigned to user terminal

Reserved 34 Reserved for future use
A single or multiple types of acknowledgment may be defined and sent on the FCCH. For example, a quick acknowledgment and an assignment-based acknowledgment may be defined. A quick acknowledgment may be used to simply acknowledge that the RACH PDU has been received by the access point but that no FCH/RCH resources have been assigned to the user terminal. An assignment-based acknowledgment includes assignments for the FCH and/or RCH for the current TDD frame.

A number of different rates are supported for the transport channels. Each rate is associated with a particular code rate and a particular modulation scheme, which collectively results in a particular spectral efficiency (or data rate). Table 15 lists the various rates supported by the system.

TABLE 15


	Spectral			Info bits/	Code bits/
Rate	Efficiency	Code	Modulation	OFDM	OFDM
Word	(bps/Hz)	Rate	Scheme	symbol	symbol

0000

0.0

—

off

—

0001	0.25	¼	BPSK	12	48
0010	0.5	½	BPSK	24	48
0011	1.0	½	QPSK	48	96
0100	1.5	¾	QPSK	72	96
0101	2.0	½	16 QAM	96	192
0110	2.5	⅝	16 QAM	120	192
0111	3.0	¾	16 QAM	144	192
1000	3.5	{fraction (7/12)}	64 QAM	168	288
1001	4.0	⅔	64 QAM	192	288
1010	4.5	¾	64 QAM	216	288
1011	5.0	⅚	64 QAM	240	288
1100	5.5	{fraction (11/16)}	256 QAM	264	384
1101	6.0	¾	256 QAM	288	384
1110	6.5	{fraction (13/16)}	256 QAM	312	384
1111	7.0	⅞	256 QAM	336	384

While the FCCH channel structure as described above can be operable at different data rates, this structure may not be efficient because the rate employed on the FCCH is dictated or limited by the worst-case user in the system (e.g., the user that operates at the lowest data rate). For example, if one of the users can only receive and decode information on the FCCH at a low data rate of 0.25 bps/Hz, other users in the system will be adversely affected even though they are capable of operating at higher data rates. This is because the rate employed on the FCCH structure will be limited to that of the worst-case user, which is 0.25 bps/Hz. Thus, the FCCH performance and efficiency may be reduced by a single user. As described in more details below, the present invention provides a novel and more efficient FCCH channel structure that can be used to accommodate different users operable at different data rates.
In one embodiment, the new FCCH structure, also referred to as a tiered control channel structure or segregated control channel structure herein), comprises multiple control channels (e.g., 4 distinct control channels). Each of these distinct control channels, also called control subchannel or FCCH subchannel herein, can operate at one of the multiple overhead data rates (e.g., one or four different data rates as mentioned above).
FIG. 7 illustrates a diagram of a new FCCH structure within a TDD MAC frame, in accordance with one embodiment of the invention. It should be understood by one skilled in the art that while TDD-TDM frame structure is used in this example for the purposes of illustration and explanation, the teachings of the present invention are not limited to TDD frame structure but can also be applied to various other frame structures of various durations (e.g., FDD-TDM, etc). As shown in FIG. 7, the TDD MAC frame is partitioned into a downlink phase (also called downlink segment) 701 and an uplink phase (also called uplink segment) 751. In this embodiment, the downlink phase is further divided into three segments for the three corresponding transport channels—the BCH 710, the FCCH 720, and the FCH 730. The uplink phase is further partitioned into two segments for the two corresponding transport channels—the RCH 740 and the RACH 750.
As shown in FIG. 7, the FCCH segment is divided or partitioned into multiple distinct FCCH segments or subchannels, each of which may operate at a specific data rate. In this example, the FCCH segment is divided into four FCCH subchannels (FCCH_0, FCCH_1, FCCH_2, and FCCH_3). In other embodiments of the invention, the FCCH segment may be divided into different numbers of subchannels (e.g., 8 subchannels, etc.), depending on the particular applications or implementations of the invention. In one embodiment, each FCCH subchannel may be associated with a specific set of operating and processing parameters (e.g., code rate, modulation scheme, SNR, etc.). For example, Table 16 below illustrates the code rates, modulation scheme, SNR, etc., that are associated with each FCCH subchannel. In this example, STTD is employed for each of the subchannels, in which case the length of each subchannel is a multiple of two OFDM symbols.

TABLE 16

FCCH Subchannel Data Rates (STTD)

Information

Bits Per Total SNR

FCCH STTD for 1%

Sub- Efficiency Code Modu- OFDM Frame Error

channel (bps/Hz) Rate lation symbol Rate (FER)

FCCH_0 0.25 0.25 BPSK 24 −2.0 dB

FCCH_1 0.5 0.5 BPSK 48 2.0 dB

FCCH_2

1 0.5 QPSK 96 5.0 dB

FCCH_3

2 0.5 16 QAM 192 11.0 dB
As shown in Table 16, each FCCH subchannel has a distinct operating point (e.g., SN_Rand other processing parameters) associated with it. A user terminal (UT) that is assigned a specific FCCH subchannel (e.g., FCCH_n at a particular rate) can correctly decode all lower rate subchannels, but not those operating at the higher rates. For example, if a particular user terminal is assigned subchannel FCCH_2, that user terminal can decode FCCH_0 and FCCH_1 subchannels because FCCH_0 and FCCH_1 operate at the lower rates. However, that user terminal cannot decode FCCH_3 because FCCH_3 operates at a higher rate. In one embodiment, the access point (AP) decides which FCCH subchannel to send control data to a UT based on various factors or selection criteria. These various factors or selection may include link quality information or operating conditions of the user terminals (e.g., C/I, Doppler, etc.), quality of service (QoS) requirements associated with the user terminals, and control subchannel preference indicated by the user terminals, etc. As described in more details below, the user terminals then attempt to decode each of the FCCH subchannels to determine if they have been allocated resources (e.g., FCH/RCH channel resources).

Table 17 illustrates the structure for the various FCCH subchannels, in accordance with one embodiment of the present invention. As shown in Table 17, the FCCH subchannel structure for subchannel FCCH_0 is distinct from the structure used for other FCCH subchannels (FCCH_1, FCCH_2, and FCCH_3). In one embodiment, the FCCH_MASK field in the FCCH_0 structure is used to indicate the presence/absence of higher rate FCCH subchannels in a particular order. For example, the FCCH_MASK field may comprise three bits each of which corresponds to a particular subchannel and is used to indicate whether the particular subchannel is present in an order from subchannel 1 (MASK bit 0), subchannel 2 (MASK bit 1), and subchannel 3 (MASK bit 2). The corresponding subchannel MASK bit is set to a particular value (e.g., 1) to indicate the presence of the respective subchannel. For example, if the value of MASK bit number 0 (the least significant MASK bit) is set to “1”, this indicates the presence of FCCH_1 subchannel. Pad bits are provided to achieve an even number of OFDM symbols in each subchannel. In one embodiment, each FCCH subchannel is capable of providing scheduling information for multiple user terminals (e.g., 32 users). The IE types described above can be used for the FCCH subchannels.

TABLE 17


FCCH Subchannel Structure

	Bits

	FCCH_0:
	FCCH MASK	3
	No. IE Rate 0	5
	Rate 0 IE's
	0 Padding
	CRC	16
	Tail	6
	FCCH_1:
	No. IE Rate 1	5
	Rate 1 IE's
	0 Padding
	CRC	16
	Tail	6
	FCCH_2:
	No. IE Rate 2	5
	Rate 2 IE's
	0 Padding
	CRC	16
	Tail	6
	FCCH_3:
	No. IE Rate 3	5
	Rate 3 IE's
	0 Padding
	CRC	16
	Tail	6

FIG. 8 illustrates a flow diagram of a method 800 in accordance with one embodiment of the present invention. At block 810, as described above, a control channel is segregated or partitioned into a plurality of subchannels each of which being operable at a specific data rate. At block 820, control information including resource allocation information is transmitted from an access point to a user terminal on a particular subchannel of the plurality subchannels selected for the user terminal, based on one or more selection criteria, as described above. At block 830, at the user terminal, one or more subchannels of the plurality of subchannels are decoded to obtain control information (e.g., channel assignments) designated for the user terminal. In one embodiment, as explained in more details below, the decoding procedure performed at the user terminal starts with the FCCH subchannel operated at the lowest data rate (FCCH_0 in this example) and continues until at least one of a plurality of conditions is satisfied.
FIG. 9 shows a flow diagram of a decoding procedure 900 performed by a user terminal in decoding the new FCCH structure, in accordance with one embodiment of the present invention. The user terminal starts by decoding the subchannel FCCH_0. In one embodiment, decoding is considered successful if the CRC test passes. The user terminal terminates FCCH decoding process whenever any of the following events occurs:

- (i) Failure to correctly decode an FCCH subchannel;
- (ii) Receipt of an assignment;
- (iii) Decoding of all active FCCH subchannels without receiving an assignment.

Referring again to FIG. 9, at block 910, the process begins by initializing n to 0. In this example, n is a variable used to indicate the current FCCH subchannel being decoded in the current iteration of the process. At block 915, the current FCCH_n subchannel is decoded. For example, in the first iteration, FCCH_0 is decoded at block 915. At block 920, it is determined whether the CRC test with respect to the current FCCH_n subchannel passes. If the CRC test passes, the process proceeds to block 925 to determine whether the corresponding MAC ID is present, otherwise the process proceeds to block 930 to process the next MAC frame. At block 925, if the corresponding MAC ID is present, the process proceeds to block 940 to obtain the assignment information provided by the access point. Otherwise, the process proceeds to block 935 to check if n is equal to 3. At block 935, if n is equal to 3, the process proceeds to block 945 to initialize the FCCH_MASK field to indicate that all FCCH subchannels have been processed. As described above, in one embodiment, the FCCH_MASK field in the FCCH_0 subchannel structure comprises three bits each of which is used to indicate the presence/absence of a corresponding higher rate FCCH subchannel. For example, the first bit (bit 0 or the least significant bit) of the FCCH_MASK field is used to indicate the presence/absence of subchannel 1, the second bit (bit 1 or the next significant bit) of the FCCH_MASK field is used to indicate the presence/absence of subchannel 2, and so on. The process then proceeds to block 950 to determine whether there are any active FCCH subchannels remaining to be decoded. If there are more active FCCH subchannels to be decoded, the process proceeds to block 960 to increment n to the next active FCCH subchannel. Otherwise the process proceeds to block 955 to process the next MAC frame.
Various parts of the MIMO WLAN system and various techniques described herein may be implemented by various means. For example, the processing at the access point and user terminal may be implemented in hardware, software, or a combination thereof. For a hardware implementation, the processing may be implemented within one or more application specific integrated circuits (ASICs), digital signal processors (DSPs), digital signal processing devices (DSPDs), programmable logic devices (PLDs), field programmable gate arrays (FPGAs), processors, controllers, micro-controllers, microprocessors, other electronic units designed to perform the functions described herein, or a combination thereof.
For a software implementation, the processing may be implemented with modules (e.g., procedures, functions, and so on) that perform the functions described herein. The software codes may be stored in a memory unit and executed by a processor. The memory unit may be implemented within the processor or external to the processor, in which case it can be communicatively coupled to the processor via various means as is known in the art.
Headings are included herein for reference and to aid in locating certain sections. These headings are not intended to limit the scope of the concepts described therein under, and these concepts may have applicability in other sections throughout the entire specification.
The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for processing information in a communication system, comprising:

partitioning a control channel used for transmitting control information into a plurality of subchannels, each subchannel being operated at a specific data rate;

selecting, for each of one or more user terminals, one of the subchannels to be used for transmitting control information from an access point to the respective user terminal, based on one or more selection criteria; and

transmitting control information from the access point to a particular user terminal on a particular subchannel selected for the respective user terminal.

2. The method of claim 1 wherein the control information is transmitted in a segment of a data frame specifically allocated for the control channel.

3. The method of claim 1 wherein each subchannel is associated with a specific set of operating parameters.

4. The method of claim 3 wherein the operating parameters are selected from the group consisting of a code rate, a modulation scheme, and a signal-to-noise ratio (SNR).

5. The method of claim 1 wherein the plurality of subchannels are transmitted sequentially in an order from a subchannel with a lowest data rate to a subchannel with a highest data rate.

6. The method of claim 5 wherein a subchannel that is transmitted first in the plurality of subchannels includes a field to indicate whether other subchannels are also being transmitted.

7. The method of claim 6 wherein the field comprises a plurality of bits each of which corresponds to a particular subchannel and is used to indicate whether the corresponding subchannel is present in the segment allocated for transmitting control information.

8. The method of claim 1 wherein the one or more selection criteria are selected from the group consisting of a first criterion corresponding to a link quality associated with the respective user terminal, a second criterion corresponding to quality of service requirements associated with the respective terminal, and a third criterion corresponding to a subchannel preference indicated by the respective terminal.

9. A method for processing information in a communication system, comprising:

segregating a control channel into a plurality of subchannels each of which being operable at a specific data rate;

transmitting control information including resource allocation information from an access point to a user terminal on particular subchannel of the plurality subchannels selected for the user terminal, based on one or more selection criteria; and

decoding, at the user terminal, one or more subchannels of the plurality of subchannels to obtain control information designated for the user terminal.

10. The method of claim 9 wherein decoding comprises:

performing a decoding procedure to decode the one or more subchannels, starting with a subchannel operated at a lowest data rate, until at least one of a plurality of conditions is met.

11. The method of claim 10 further comprising:

terminating the decoding procedure if one of the plurality of conditions is met.

12. The method of claim 11 wherein the plurality of conditions includes a first condition indicating a failure to correctly decode one of the plurality of subchannels.

13. The method of claim 11 wherein the plurality of conditions includes a second condition indicating that control information designated for the user terminal has been obtained from one of the plurality of subchannels.

14. The method of claim 11 wherein the plurality of conditions includes a third condition indicating that all subchannels have been processed.

15. The method of claim 10 wherein performing a decoding procedure comprises:

determining whether information transmitted on a subchannel has been correctly received, based on a quality metric corresponding to the respective subchannel.

16. The method of claim 15 wherein the quality metric comprises a cyclic redundancy check (CRC).

17. The method of claim 10 wherein performing a decoding procedure comprises:

determining whether control information designated for the user terminal is present in the respective subchannel, based on an identifier associated with the user terminal.

18. The method of claim 17 wherein the identifier comprises a Medium Access Control (MAC) identifier.

19. The method of claim 9 wherein the one or more selection criteria are selected from the group consisting of a first criterion corresponding to operating conditions of the respective user terminal, a second criterion corresponding to quality of service requirements associated with the respective terminal, and a third criterion corresponding to a subchannel preference indicated by the respective terminal.

20. An apparatus for processing information in a communication system, comprising:

means for partitioning a control channel that is used for transmitting control information into a plurality of subchannels, each subchannel being operated at a specific data rate;

means for selecting, for each of one or more user terminals, one of the subchannels to be used for transmitting control information from an access point to the respective user terminal, based on one or more selection criteria; and

means for transmitting control information from the access point to a particular user terminal on a particular subchannel selected for the respective user terminal.

21. The apparatus of claim 20 wherein each subchannel is associated with a distinct set of operating parameters including a code rate, a modulation scheme, and an SNR.

22. The apparatus of claim 20 wherein the plurality of subchannels are transmitted sequentially in an order from a subchannel with a lowest data rate to a subchannel with a highest data rate.

23. The apparatus of claim 22 wherein a subchannel that is transmitted first in the plurality of subchannels includes a field to indicate whether other subchannels are also being transmitted.

24. The apparatus of claim 20 wherein the one or more selection criteria including a first criterion corresponding to a link quality associated with the respective user terminal, a second criterion corresponding to quality of service requirements associated with the respective terminal, and a third criterion corresponding to a subchannel preference indicated by the respective terminal.

25. An apparatus for processing information in a communication system, comprising:

means for segregating a control channel into a plurality of subchannels each of which being operable at a specific data rate;

means for transmitting control information including resource allocation information from an access point to a user terminal on particular subchannel of the plurality subchannels selected for the user terminal, based on one or more selection criteria; and

means for decoding, at the user terminal, one or more subchannels of the plurality of subchannels to obtain control information designated for the user terminal.

26. The apparatus of claim 25 wherein means for decoding comprises:

means for performing a decoding procedure to decode the one or more subchannels, starting with a subchannel operated at a lowest data rate, until at least one of a plurality of conditions is met.

27. The apparatus of claim 26 wherein the plurality of conditions includes a first condition indicating a failure to correctly decode one of the plurality of subchannels, a second condition indicating that control information designated for the user terminal has been obtained from one of the plurality of subchannels, and a third condition indicating that all subchannels have been processed.

28. The apparatus of claim 25 wherein means for performing a decoding procedure comprises:

means for determining whether information transmitted on a subchannel has been correctly received, based on a quality metric corresponding to the respective subchannel; and

means for determining whether control information designated for the user terminal is present in the respective subchannel, based on an identifier associated with the user terminal.

29. The apparatus of claim 25 wherein the one or more selection criteria including a first criterion corresponding to operating conditions of the respective user terminal, a second criterion corresponding to quality of service requirements associated with the respective terminal, and a third criterion corresponding to a subchannel preference indicated by the respective terminal.

30. An apparatus for processing information in a communication system, comprising:

a controller configured to select one of a plurality of control subchannels to send control information to a user terminal, based on one or more selection criteria, each subchannel being operable at a specific data rate; and

a transmitter to send the control information designated for the user terminal on the subchannel selected for the user terminal.

31. The apparatus of claim 30 wherein each subchannel is associated with a specific set of operating parameters, including a data rate at which control information is transmitted, a code rate, a modulation scheme, and an SNR.

32. The apparatus of claim 30 wherein the plurality of subchannels are transmitted sequentially in an order from a subchannel with a lowest data rate to a subchannel with a highest data rate.

33. The apparatus of claim 30 wherein the one or more selection criteria including a first criterion corresponding to a link quality associated with the respective user terminal, a second criterion corresponding to quality of service requirements associated with the respective terminal, and a third criterion corresponding to a subchannel preference indicated by the respective terminal.

34. An apparatus for processing information in a wireless communication system, comprising:

a receiver to receive information on one or more control subchannels each of which being operated at a specific data rate; and

a decoder to decode the one or more control subchannels to obtain control information designated for a particular user terminal, starting with a subchannel operated at a lowest data rate, until at least one of a plurality of conditions is met.

35. The apparatus of claim 34 wherein the plurality of conditions includes a first condition indicating a failure to correctly decode one of the plurality of subchannels, a second condition indicating that control information designated for the user terminal has been obtained from one of the plurality of subchannels, and a third condition indicating that all subchannels have been processed.

36. The apparatus of claim 34 wherein the decoder is configured to determine whether information transmitted on a subchannel has been correctly received, based on a quality metric corresponding to the respective subchannel and to determine whether control information designated, for the user terminal is present in the respective subchannel, based on an identifier associated with the user terminal.

37. A machine-readable medium comprising instructions which, when executed by a machine, cause the machine to perform operations including:

partitioning a control channel that is used for transmitting control information into a plurality of subchannels, each subchannel being operated at a specific data rate;

38. The machine-readable medium of claim 37 wherein each subchannel is associated with a set of operating parameters, including a data rate at which control information is transmitted, a code rate, a modulation scheme, and an SNR.

39. The machine-readable medium of claim 37 wherein the one or more selection criteria including a first criterion corresponding to a link quality associated with the respective user terminal, a second criterion corresponding to quality of service requirements associated with the respective terminal, and a third criterion corresponding to a subchannel preference indicated by the respective terminal.

40. A machine-readable medium comprising instructions which, when executed by a machine, cause the machine to perform operations including:

receiving information on one or more control subchannels each of which being operated at a specific data rate; and

decoding the one or more control subchannels to obtain control information designated for a particular user terminal, starting with a subchannel operated at a lowest data rate, until at least one of a plurality of conditions is met.

41. The machine-readable medium of claim 40 wherein the plurality of conditions includes a first condition indicating a failure to correctly decode one of the plurality of subchannels, a second condition indicating that control information designated for the user terminal has been obtained from one of the plurality of subchannels, and a third condition indicating that all subchannels have been processed.

42. The machine-readable medium of claim 40 wherein the decoder is configured to determine whether information transmitted on a subchannel has been correctly received, based on a quality metric corresponding to the respective subchannel and to determine whether control information designated for the user terminal is present in the respective subchannel, based on an identifier associated with the user terminal.

43. A method for processing information in a system, comprising:

44. The method of claim 43 wherein the plurality of conditions includes a first condition indicating a failure to correctly decode one of the plurality of subchannels, a second condition indicating that control information designated for the user terminal has been obtained from one of the plurality of subchannels, and a third condition indicating that all subchannels have been processed.

45. The method of claim 43 wherein decoding comprises:

determining whether information transmitted on a subchannel has been correctly received, based on a quality metric corresponding to the respective subchannel; and